This invention relates to improved structural elements for metal structural work, which have the advantages both of tubular elements (mainly, shape stability and torsional rigidity) and of open sections (mainly, low production costs, easiness of connection, convenience in transportation).
The present invention also relates to structures, generally flat and three-dimensional girders, and in particular supports for overhead electric power transmission lines, which can be constructed in a particularly advantageous manner with said improved structural elements.
For the construction of supports for overhead electric power transmission lines, structural elements are currently used consisting essentially of open sections (usually angle irons), which are favored for their low production costs, easiness of connection, easy protection treatments (galvanising) and reduced transportation bulk.
However these elements, which are well known and widely spread, give rise to local or torsional instability problems when used as bars subjected to combined compressive and bending stresses, such problems being solved by using sections which are thicker and heavier than desirable.
Moreover, flat or slender structures constructed with these conventional elements have very low critical out-of-plane twisting loads, a consequence of the torsional instability of the component parts, and this can be a serious limitation in the planning of certain advanced types of supports for electric lines.
All these drawbacks could easily be eliminated by replacing the structural elements in the form of open sections by tubular metal elements. However these latter, though having properties which have long been known and appreciated, have had litte application in practice because of the high production costs of the tubes, the difficulty of making the connections and the consequent high costs of these latter, and the considerable transportation bulk thereof.